The present invention is directed to an explosive powder charge operated fastening element setting tool with a hydropneumatic damping unit, a housing for guiding a driving piston, and a handle part displaceable in the setting direction relative to the housing against the force of the hydropneumatic damping unit. The handle part has a space for a cylinder containing a liquid medium under gas pressure and the housing cooperates with a piston displaceable in the cylinder against the flow resistance of the liquid medium, and the piston divides the cylinder into a front cylinder space and a rear cylinder space.
Explosive powder charge operated setting tools use a driving piston powered by the expanding gasses of an ignited propellant charge which drives the fastening element to be driven. Usually such setting tools have a relatively large rebound, therefore, in the past effort has been made to dampen the rebound to make the tool more comfortable for use by an operator.
An explosive powder charge operated fastening element setting tool is disclosed in U.S. Pat. No. 2,731,636 which has a guide housing for the driving piston guide. A spring is provided between the guide housing and a handle displaceable relative to it and is meant to absorb and dampen the rebound or recoil acting on the tool operator when a fastening element is being driven. The damping effect of such a spring is sufficient only at low spring rates. A low spring rate necessitates a large spring and a larger space for housing the spring, so that a tool equipped with such an arrangement is difficult to handle. Using a smaller spring with a higher spring rate lowers the damping effect, whereby a high share of the undesirable rebounding forces still act on the tool operator. Further, the restoring velocity in such known springs is very high, so that during the restoring operation undesirable shocks act on the tool operator.
To avoid these problems it is proposed in EP-A-0 331 168 to use a hydropneumatic damping unit with valve systems. In such a tool the recoil is absorbed by the hydropneumatic damping unit formed by a cylinder containing a liquid medium under gas pressure and a piston located within the cylinder and displaceable opposite to the flow resistance of the liquid medium. The cylinder is supported in a housing part connected to the guide part. A shaft connected to the piston protrudes from the cylinder and abuts the handle part. If the handle part and the guide housing are displaced relative to one another, the displacement force is transmitted to the piston by the shaft. The piston divides the cylinder into a front cylinder space and a rear cylinder space. The liquid medium to which pressure is applied during the movement of the piston, flows through the valves from the rear into the front cylinder space and in the opposite direction. The valve system using butterfly valves controls the flow apertures for the liquid medium cylinder depending on the direction of movement of the piston. When the piston moves into the liquid medium, the aperture cross-sections of the valves are larger than the when the piston moves in the opposite direction. Therefore, the damping effect is greater when the piston is moved back out of the medium than during the driving process and the piston returns slowly into its original position. Without such control of the aperture cross-section of the valves, the piston would bounce back in a springlike manner which can result in undesirable shocks affecting the tool operator.
Extreme shock loads can occur in operation due to the explosion-like expansion of propellant gases in the tool. Accordingly, shockwaves can be transmitted through the shaft to the damping arrangement. Particularly, the butterfly vlves of the valve system are exposed to considerable loads and can be destroyed limiting the damping effect. In an extreme situation, the aperture cross-section of the valves is no longer reduced when the piston is pulled back and damping is not increased and the piston rebounds in a resilient, springy fashion.